Alkylated derivatives of diphenylamines are well known compounds that are commonly used as antioxidants for lubricating oils, natural and synthetic rubbers and plastics. Alkylated diphenylamine compounds include, for example; 4,4'-bis(.alpha.,.alpha.-dimethylbenzyl)diphenylamine as described in U.S. Pat. No. 3,505,225; 2,2'-diethyl-4,4'-tert-dioctyldiphenylamine as described in U.S. Pat. No. 3,732,167; 2,2',4,4'-tetra-t-butyldiphenylamine as described in U.S. Pat. No. 3,655,559; and p,p'-di-tertiary-octyl-diphenylamine and p,p'-di-(.alpha.-phenylethyl)diphenylamine as described in U.S. Pat. No. 2,530,769. Alkylated diphenylamines are typically prepared by alkylating a diphenylamine with olefins such as ethylene, heptene, octene, nonene, styrene, and diisobutylene, in the presence of a suitable alkylation catalyst. For example, U.S. Pat. No. 2,530,769 discloses the para-alkylation of diphenylamines using various olefins and a Friedel-Crafts condensation catalyst such as aluminum chloride, and U.S. Pat. No. 2,943,112 discloses the alkylation of diphenylamine with heptenes, octenes, and nonenes using a Filtrol Clay as a catalyst.
As to the ortho-alkylation step, an article in the "Journal of Organic Chemistry", Vol. 21, page 711 (1956) discloses a process and mechanism for ortho-alkylating aromatic amines using aluminum metal to form an aluminum anilide catalyst. U.S. Pat. Nos. 2,762,845 and 2,814,646 also discloses the use of aluminum metal to form an aluminum anilide to effect the ortho-alkylation of aromatic amines. The U.S. Pat. No. 2,814,646 also discloses that aluminum halide used with an alkali or alkaline earth metal anilide can be used in the ortho-reaction. U.S. Pat. No. 3,923,892 discloses the use of an alkylaluminum halide with aromatic amines to achieve accelerated reaction rates. An article in the German journal Angewandte Chemie, Volume 69, page 124 (1957) discloses the ortho-alkylation of diphenylamine using as a catalyst the reaction product of aluminum metal with aniline.
Ortho-para-alkylated diphenylamines are typically synthesized in a two-stage process. In the first stage, diphenylamine is alkylated at one or both of the ortho-(2 and 2') positions by reacting with a suitable olefin and typically using an aluminum catalyst. The ortho-alkylated product is then isolated, generally by fractional distillation or by washing the crude mixture with water, and subsequently para-alkylated at one or both of the para-(4 and 4') positions with additional olefin. The U.S. Pat. No. 3,655,559 referenced above describes such a general process, wherein the ortho-alkylation is first performed by reacting diphenylamine with a 2 to 4 carbon olefin, and the para-alkylation involves the subsequent reaction of the ortho-alkylated diphenylamine with a secondary olefin having 4 to 12 carbons, such as isobutylene, 2-methyl pentene-1, diisobutylene and propylene trimer. Although the U.S. Pat. No. 3,655,559 discloses a preparation of a tetra-substituted product in a single-stage alkylation where the ortho- and para-substituents are the same t-butyl groups, for different alkyl substituents at the ortho- and para-positions the two-stage alkylation is disclosed.
The necessity for isolation of the ortho-alkylated diphenylamine intermediate prior to the para-alkylation introduces an undesirable step with attendant disadvantages. The isolation step results in additional process equipment requirements, longer preparation times, lower productivity, increased catalyst usage, and increased costs. An alkylation process which can effectively produce ortho-para-alkylated diphenylamines having different ortho- and para-substituents, without the necessity for isolation of the ortho-alkylated intermediate, is most desirable.
It is an object of the present invention to provide an improved process for ortho-para-alkylation of diphenylamines wherein the intermediate product does not have to be isolated. It is a further object of this invention to achieve a more efficient process for the preparation of ortho-para-alkylated diphenylamines where the ortho- and para-substituents are different entities.